Page 1 of 10 DK or Dielectric Constant or Relative Permittivity or r What is it, Why is it Important, and How Does Taconic Test for It? By David L. Wynants, Sr. Process Engineer, Taconic ADD The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. Technically, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum. Similarly, it is also the ratio of the capacitance of a capacitor using that material as a dielectric, compared to a similar capacitor which has vacuum, or air, as its dielectric 1 . In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium, e.g, a dielectric such as a laminate or film. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how much electric field (more correctly, flux) is 'generated' per unit charge. Less electric flux exists in a medium with a high permittivity (per unit charge) due to polarization effects. Permittivity is directly related to electric susceptibility, which is a measure of how easily a dielectric polarizes in response to an electric field. Thus, permittivity relates to a material's ability to transmit (or "permit") an electric field 2 . The dielectric constant (DK) is an essential piece of information when designing capacitors and in other circumstances where a material might be expected to introduce capacitance into a circuit. The layers beneath etched conductors in printed circuit boards (PCBs) also act as dielectrics. Dielectrics are used in RF transmission lines. Electrical signals on wires and traces travel at the speed of light: 186,280 miles/second! That works out to 11.8 in/nanosecond. Electrical signals slow down in any other medium by the square root of the relative dielectric coefficient of the medium. So, for example, a stripline trace in FR4 with an r of 4.0 would travel at the speed of light divided by the square root of 4 (which is 2) or about 6 in/ns. This is valid in a stripline or multilayer application where all the flux lines are going through materials having the same or similar DK. In a microstrip application (such as in a double sided board, or on the outer layer of a multilayer board), some part of those flux lines travel in air, so the effective dielectric constant will be slightly less. Software programs take this into account when designing those types of circuits 3 . In physics, the dissipation factor (DF) is a measure of loss-rate of energy of a mode of oscillation (mechanical, electrical, or electromechanical) in a dissipative system. It is the reciprocal of Quality factor, (Q) which represents the quality of oscillation. For example, electrical potential energy is dissipated in all dielectric materials, usually in the form of heat. When representing the electrical circuit parameters as vectors in a complex plane, to the right, the dissipation factor is equal to the tangent of the angle between the impedance vector and the negative reactive axis, as shown in the diagram to the right. This gives rise to the parameter known as the loss tangent δ, or tan delta. DF will vary depending on the dielectric material and the frequency of the electrical signals.
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Page 1 of 10
DK or Dielectric Constant or Relative Permittivity or r
What is it, Why is it Important, and How Does Taconic Test for It?
By David L. Wynants, Sr. Process Engineer, Taconic ADD
The relative permittivity of a material under given conditions reflects the extent to which it concentrates
electrostatic lines of flux. Technically, it is the ratio of the amount of electrical energy stored in a material by an
applied voltage, relative to that stored in a vacuum. Similarly, it is also the ratio of the capacitance of a
capacitor using that material as a dielectric, compared to a similar capacitor which has vacuum, or air, as its
dielectric1.
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming
an electric field in a medium, e.g, a dielectric such as a laminate or film. In other words, permittivity is a
measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium
describes how much electric field (more correctly, flux) is 'generated' per unit charge. Less electric flux exists in
a medium with a high permittivity (per unit charge) due to polarization effects. Permittivity is directly related to
electric susceptibility, which is a measure of how easily a dielectric polarizes in response to an electric field.
Thus, permittivity relates to a material's ability to transmit (or "permit") an electric field2.
The dielectric constant (DK) is an essential piece of information
when designing capacitors and in other circumstances where a
material might be expected to introduce capacitance into a circuit.
The layers beneath etched conductors in printed circuit boards
(PCBs) also act as dielectrics. Dielectrics are used in RF
transmission lines.
Electrical signals on wires and traces travel at the speed of light:
186,280 miles/second! That works out to 11.8 in/nanosecond.
Electrical signals slow down in any other medium by the square root of the relative dielectric coefficient of the
medium. So, for example, a stripline trace in FR4 with an r of 4.0 would travel at the speed of light divided by
the square root of 4 (which is 2) or about 6 in/ns. This is valid in a stripline or multilayer application where all
the flux lines are going through materials having the same or similar DK. In a microstrip application (such as in
a double sided board, or on the outer layer of a multilayer board), some part of those flux lines travel in air, so
the effective dielectric constant will be slightly less. Software
programs take this into account when designing those types of
circuits3.
In physics, the dissipation factor (DF) is a measure of loss-rate of
energy of a mode of oscillation (mechanical, electrical, or
electromechanical) in a dissipative
system. It is the reciprocal of Quality
factor, (Q) which represents the
quality of oscillation. For example, electrical potential energy is dissipated in all
dielectric materials, usually in the form of heat.
When representing the electrical circuit parameters as vectors in a complex
plane, to the right, the dissipation factor is equal to the tangent of the angle
between the impedance vector and the negative reactive axis, as shown in the
diagram to the right. This gives rise to the parameter known as the loss tangent
δ, or tan delta. DF will vary depending on the dielectric material and the
As DF is an indication of power, let’s discuss dB briefly. The term dBm is an abbreviation for the power ratio
in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and
fiber optic networks as a convenient measure of absolute power because of its capability to express both very
large and very small values. Zero dBm equals one milliwatt. A 3 dB increase represents roughly doubling the
power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one
half, making −3 dBm equal to about 0.5 milliwatt5. The -3 dBm frequencies are used in determining the DF of a
material in some of the test methods for DK we’ll be discussing.
Presented below are discussions of the four DK test methods we do at Taconic, Petersburgh [TP]. Three of them
are IPC methods, listed as a DK test method for all 17 legacy data sheets in IPC 4130A. One was co-opted by
someone connected to IPC, into designing the IPC-TM-650 2.5.5.5.1 method.
Two Fluid Cell Method [@ 1 MHz] IPC-TM-650 2.5.5.3
As the definition of DK is a ratio of capacitances, and this method measures capacitance, this method excels at
DK. The ratios are that of an empty cell [air as the fluid] without and with the material under test (MUT), and a
wet cell [Dow 200 silicone fluid] without and with the MUT. It is a destructive test since a discrete sized sample
[~3X3] is required.
But at 1 MHz? What’s the use? Well, if the DK of the material doesn’t change with frequency, as with PTFE,
then the Two Fluid Cell test is as valid as a test at 10 GHz. In addition, the Two Fluid Cell Method is easier to
perform, and multiple tests of the same dielectric thickness (DT) can be performed nearly simultaneously.
Page 3 of 10
Virtually any DK can be measured using this method.
Speaking of DTs: From the very thin to the very thick [~0.0001” to 0.2500”] can be tested. So the actual
material that customers are buying is being tested. This is an important advantage for Taconic.
The DF, in my opinion, is worthless however. Why? Very thin materials have an extremely high tested DF,
while very thick samples with the same DK exhibit very low DFs. Correlations of measured DF between the
Two Fluid Cell method and other test methods are non-existent, while the DK obtained by the Two Fluid Cell
correlates well with the other two DK test methods that IPC references on all 17 legacy slash sheets of IPC
4103. Notice the excellent relationship between the DKs and the non-existent correlation of the DFs done with
the Two Fluid Cell and the X-Band test, as shown in the charts above.
The temperature of the laboratory environment, or fixture, needs to be well controlled. The lab cannot get too
hot or the DK will be too low. This is typical PTFE behavior. Chilling the fixture can remediate potential
temperature issues within the lab. Some porous, highly filled, ceramic products can absorb the silicone fluid and
be problematic to test, since the capacitance raises as the air is displaced by the fluid, seemingly increasing the
DK. Values need to be recorded immediately upon entry of the MUT in those instances.
Figure 1. Not-to-Scale Schematic of 2-Fluid Cell fixture. Readings are taken with the cell empty, then with the MUT inserted. The MUT is taken out, the fixture is filled with Dow 200 silicone fluid. Readings are taken; the MUT is re-inserted and final readings taken. A spreadsheet spits out the DK.
Here is a link to the IPC test method: http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2.5.5.3c.pdf
Full Sheet Resonance [FSR] Method [TP Stnd @ 130 to 500 MHz] IPC-TM-650 2.5.5.6
FSR is a non-destructive test and faster to perform than the Two Fluid Cell method. Given a clean-cut sample
edge, and sufficient floor space, virtually any sized panel can be tested, from a 6X6 to an 18X102.
What happens in an FSR test? A signal is launched from the edge of a cut panel. The clad panel acts as a wave
guide. The edges behave as “opens” and so reflect the signal back into the panel. The DK and the panel
dimension determine how the reflected signals behave inside the panel to create the resonant pattern seen on the